In a conventional valve device, a concave spherical seat face of a valve seat is pressed onto a convex spherical ball face of a ball valve to rotate the ball valve, thereby switching communication and non-communication between a valve opening of the ball valve and a seat opening of the valve seat (see, for example, Patent Literature 1).
In the conventional valve device, a diameter of the seat opening matches a diameter of the valve opening. Alternatively, the diameter of the seat opening is larger than the diameter of the valve opening. The diameter of the opening is an inner diameter of each opening.
The seat face of the valve seat is pressed onto the ball face of the ball valve. The ball valve rotates at opening/closing and adjustment of the degree of opening. Thus, the ball face and the seat face slide in contact with each other under pressure.
Here, the ball face of the ball valve moves with respect to the seat face. For this reason, a site of the ball face in contact with the seat face at opening is different from a site of the ball face in contact with the seat face at closing. On the contrary, since the seat face does not move, the site of the seat face in contact with the ball face at opening is the same as the site of the seat face in contact with the ball face at closing.
That is, at closing of the valve, the ball face contacts the seat face only at closing, while the seat face contacts the ball face continuously from closing to full opening. Thus, the site of the ball face in contact with the seat face at closing is hard to wear.
On the contrary, the site of the seat face in contact with the ball face at closing slides on the ball face at all times and thus, is susceptible to wear. For this reason, when the valve device is used for a long time, the seat face that ensures a sealing property at closing wears earlier than the ball face that ensures the sealing property at closing, possibly causing leakage at closing.
The valve seat in Patent Literature 1 is assembled to a holding member using a cylindrical portion attached to the periphery of the valve seat. Since the cylindrical portion in Patent Literature 1 is an individual component for attaching the valve seat to the holding member, the number of components increases.
Further, since the cylindrical portion in Patent Literature 1 forms an annular groove receiving the valve seat, the axial length of the cylindrical portion is larger than the thickness of the valve seat. As described above, in the valve device having the conventional configuration, the valve seat is susceptible to wear. For this reason, when the valve seat continues to wear and becomes thinner, the ball valve may directly contacts the cylindrical portion, eliminating the sealing effect of the valve seat.